Zein is a principal component of corn protein. Because of its various excellent characteristics, a number of applications or uses of zein have been developed. However, there is an increasing demand for a purified white zein product free from undesirable color and odor, which can therefore be used in food and cosmetics.
Conventional processes for the production of zein include: (1) a process in which zein is extracted using a water-containing solvent, and the resulting extract solution is cooled to precipitate and recover zein while pigment and odor components remain dissolved in the extract solution (see JP-B-16800, the term "JP-B" as used herein means an "examined Japanese patent publication"); (2) a process in which zein is extracted using water-containing ethanol under weak alkaline conditions, concentrated at reduced pressure and then contacted with cool water to recover it as a water insoluble component (see JP-A-63-185999, the term "JP-A" as used herein means an "unexamined published Japanese patent application"); and (3) a process in which a zein solution is contacted with acetone, cold alcohol and the like, and the resulting insolubilized zein is recovered and dried (see U.S. patent application No. 07/874,864 filed on Apr. 28, 1992 now U.S. Pat. No. 5,342,923).
In the first process, zein is precipitated due to its lack of solubility in solution. However, a solvent having a high ability to extract zein (i.e., a high water content solvent) cannot insolubilize and precipitate zein selectively during the cooling step and may cause insolubilization of not only zein but also pigment and oil fractions, thus causing difficulty in obtaining purified zein. This process is also disadvantageous in that partly dissolved zein remaining in the supernatant fluid causes yield loss and the precipitation step requires a prolonged period of time.
In the second process, zein may be subject to thermal denaturation during the concentration of the zein solution extracted with the water-containing solvent and during precipitation and drying due to a relative increase in the water content of the precipitate.
In the third process, pigment and oil components can be removed from zein, but the process cannot be practically used for the selective recovery of pigment.
On the other hand, processes have been proposed by the inventors of the present invention in U.S. Pat. application No. 07/868,907 filed on Apr. 16, 1992, now U.S. Pat. No. 5,367,055 and JP-A-5-30919 conceiving the removal of pigment. In these processes, the pigment components, which are difficult to extract, are extracted using acetone and alcohol, which contain water in a small amount such that zein is not extracted, and a mixed solvent thereof and by allowing the corn gluten meal to swell slightly. This, however, is a treatment process for obtaining pigment-free zein and not a recovery process for obtaining practically useful pigment. That is, since whole oil and fat components are extracted together with the pigment, the resulting product has a thin color density and therefore cannot be put into practical use as a pigment preparation for use in food, cosmetics and the like.
When extraction of zein from corn gluten meal is carried out using a solvent having a relatively high water content (for example, an aqueous solution of less than 91% ethanol), the swelling degree and other properties of the corn gluten meal during the extraction vary depending on the quality of the corn gluten meal used, thus causing great variation in the properties of the extracted solution and altering the operation of the filtration and centrifugation steps. Because of this, it becomes necessary to control the extraction conditions depending on the properties of each corn gluten meal material.
This invention has been made, taking the aforementioned problems involved in the conventional zein production processes into consideration, with the aim of providing a process in which treatment conditions during extraction of zein are stabilized and decolorized and purified zein is provided through continuous treatment steps, as well as a process for the recovery by concentration of corn pigment having a bright yellow color, the pigment being useful in food and the like.
In view of the above, the inventors of the present invention have conducted intensive studies on the development of a process in which treatment conditions during the extraction of zein are stabilized so that continuous treatment can be conducted, and a process by which production of zein and concentration of pigment can be made simultaneously. The inventors have found a process by which extraction of zein can be made without causing swelling of corn gluten meal, the processes being carried out by the use of water-containing alcohol and water-containing acetone having lower water contents in comparison with those in conventional extraction processes at a higher temperature in comparison with the conventional process and by the separation of residual materials. According to the invention, extraction of zein can be carried out within a short period of time without causing deterioration of product quality, the product not being influenced by the quality of corn gluten meal used as the starting material.
In the extraction process of the present invention, however, oils and fats and impurities may also be extracted in high quantities together with zein because of the low water content of the extraction solvent and high extraction temperature in comparison with conventional processes. Because of this, when zein is precipitated and recovered by cooling the extract solution, a small amount of a water-containing solvent may be added to the precipitate to separate insoluble components by filtration. Oil and fat components remain in the solution and form minute oil droplets during the filtration separation step. Being minute in size, these oil droplets have a tendency to pass through a filter together with the solution. On the other hand, when an extremely fine mesh filter, such as an ultrafiltration membrane, is used to prevent such a problem, it causes another problem in that filtration resistance and blinding (the meshes of filter are stopped up) occur, thus entailing extremely reduced filtration efficiency. When oils and fats remain in the filtration step, it also causes a problem in that the pigment component dissolves in oils and fats remaining in the zein precipitate and causes dense coloring of zein, thus accelerating the change in quality of a zein solution.
In consequence, the inventors of the present invention have further examined these problems and found that, when a pretreatment step is employed using a solvent discussed below, oils and fats which have caused problems in the subsequent purification steps can be removed in advance from corn gluten meal, and the pigment remains in a relatively high quantity in the treated corn gluten meal. On the other hand, examination was made also of the use of gluten meal from which oil and fat and pigment components have been removed by the process disclosed in U.S. Pat. application No. 07/868,907 filed on Apr. 16, 1992 and JP-A-5-30919 in which acetone and alcohol which contain water in a reduced amount, such that zein is not extracted, and a mixed solvent thereof is used.